1. Field of the Invention
The present invention relates to an addition salt of an optically active acyl-amino acid and an optically impure .alpha.-arylamine, and a method of optical resolution using same.
2. Discussion of the Background
Optically active .alpha.-arylalkylamines are important materials as optical resolution agents to obtain optically active substance from a racemic carboxylic acid. Among them, amines wherein the aryl group is phenyl and the alkyl group is methyl or ethyl have been extensively used as optical resolution agents. Further, amines wherein the aryl group is phenyl or methyl-substituted phenyl and which is an S-isomer are important as a raw material for high-potency sweeteners disclosed in U.S. Pat. No. 5,286,509.
To obtain an optically active .alpha.-arylakylamine from an optically impure mixture by optical resolution, a method is, generally used wherein salts with various optically active carboxylic acids are formed and the resulting two diastereomeric salts are separated utilizing differences in solubility. Several methods are known, for example, a method utilizing optically active tartaric acid or malic acid (J. Chem. Soc., 1940, 336), a method utilizing optically active N-acetyl-3,5-dibromo-tyrosine (J. Am. Chem. Soc., 73, 5782 (1951)), a method utilizing optically active 2-benzamidocyclohexanecarboxylic acid (Bull. Chem. Soc. Jpn., 61, 1395 (1988)).
However, such methods utilizing tartaric acid or malic acid have poor optical purification ability, and the resulting diastereomeric salts should be repeatedly crystallized for purification. While tartaric acid and malic acid are relatively cheap, they are difficult to recover efficiently from such separation operation, which becomes a problem for industrialization.
The production of optically active N-acetyl-3,5-dibromo-tyrosine has the drawbacks of involving troublesome operations and has poor optical purification ability.
When optically active 2-benzamidocyclohexanecarboxylic acid is used, an amine of high optical purity may be obtained by a single operation of crystallization. However, crystallization yields are not so high. Further, this material is relatively expensive.
From these reasons, while the conventionally known resolution agents are excellent on a laboratory scale, there are some problems of application on an industrial scale.
The problem addressed by the invention is to develop an industrial process to produce, efficiently and at a low cost, optically active .alpha.-arylalkylamines from optically impure .alpha.-arylalkylamines by optical resolution.
The present inventors have studied intensively to solve the above problems. Surprisingly, we have found that optically active N-acyl-aspartic acid (aspartic acid is hereinafter abbreviated to Asp) and optically impure .alpha.-arylalkylamine are mixed in a suitable solvent to give a salt of the optically active N-acyl-Asp and equimolar optically active e-arylalkylamine separated out. Further, we have found that similar optical resolution effect is obtained for optically active N-acyl-glutamic acid (glutamic acid is hereinafter abbreviated to Glu). Thus, we have attained the present invention.